Cellular telephones are becoming more and more popular. According to the Cellular Telecommunications Industry Association, there were over 97 million cellular telephones in use in the United States alone in 1999. As more and more people become “cell phone active”, they begin to demand more and more features from their telephones.
Numerous products on the market today incorporate non-traditional features into cellular phones. For example, the StarTac™ series phone manufactured by Motorola can be purchased with a clip-on personal data assistant, which is capable of storing appointments, phone lists, and to-do reminders. Additionally, the Visor™ personal data assistant manufactured by Handspring has an expansion slot that allows it to become a cellular telephone.
One such accessory for cellular telephones that has been recently incorporated is a MP3 player. “MP3” is an acronym that stands for MPEG-1 Audio Layer-3, which is a standard technology format for compressing digital music into a small file format. MP-3 technology allows large amounts of information to be stored in small, relatively inexpensive memory chips. Many cellular phone manufacturers, including Motorola, will soon offer clip-on devices that allow people to play MP-3 music through their cellular phone.
There is a challenge in this design, however, in that cellular phone users want to use the same speaker or headphones for MP-3 music as they do for cellular calls. In other words, people want their headset to automatically switch from phone call to MP-3 music, without having to adjust the headset. This creates a problem in that MP-3 music is in stereo, which means that the music channel for the left ear and the music channel for the right ear are separate. Cellular calls, on the other hand are mono, meaning that the same channel is fed to both ears. Thus, any automatic switching circuit must provide for this difference. Additionally, there is great pressure to keep the cost of electronic products low. Thus, any solution must have a small part count.
A need therefore exists for a system capable of switching between telephone calls and other audio accessories in cellular telephones.